Which element isn't an important nutrient contained in commercial fertilizers?

Commercial fertilizers are typically rich in essential nutrients that plants require for growth, with the three primary macronutrients being nitrogen, phosphorus, and potassium. Each of these plays a vital role in plant health: nitrogen supports leaf growth and development; phosphorus contributes to root, flower, and fruit development; and potassium aids in overall plant functions, including water regulation and enzyme activation.

Oxygen, while critical to plant respiration, is not included as a nutrient in fertilizers. Plants obtain oxygen primarily from the air and through their root systems in the soil, making it unnecessary for fertilizers to contain oxygen as an added nutrient. Thus, the element that is not an important nutrient in commercial fertilizers is oxygen.